Heavy oil is naturally formed oil with very high viscosity but often contains impurities such as sulfur. While conventional light oil has viscosities ranging from about 0.5 centipoise (cP) to about 100 cP, heavy oil has a viscosity that ranges from 100 cP to over 1,000,000 cP. Heavy oil reserves are estimated to equal about fifteen percent of the total remaining oil resources in the world. In the United States alone, heavy oil resources are estimated at about 30.5 billion barrels and heavy oil production accounts for a substantial portion of domestic oil production. For example, in California alone, heavy oil production accounts for over sixty percent of the states total oil production. With reserves of conventional light oil becoming more difficult to find, improved methods of heavy oil extractions have become more important. Unfortunately, heavy oil is typically expensive to extract and recovery is much slower and less complete than for lighter oil reserves. Therefore, there is a compelling need to develop a more efficient and effective means for extracting heavy oil.
Viscous oil that is too deep to be mined from the surface may be heated with hot fluids or steam to reduce the viscosity sufficiently for recovery by production wells. One thermal method, known as steam assisted gravity drainage (SAGD), provides for steam injection and oil production to be carried out through separate wellbores. The optimal configuration is an injector well which is substantially parallel to and situated above a producer well, which lies horizontally near the bottom of the formation. Thermal communication between the two wells is established and, as oil is mobilized and produced, a steam chamber or chest develops. Oil at the surface of the enlarging chest is constantly mobilized by contact with steam and drains under the influence of gravity.
There are several patents on the improvements to SAGD operation. U.S. Pat. No. 6,814,141 describes applying vibrational energy in a well fracture to improve SAGD operation. U.S. Pat. No. 5,899,274 teaches addition of solvents to improve oil recovery. U.S. Pat. No. 6,544,411 describes decreasing the viscosity of crude oil using ultrasonic source. U.S. Pat. No. 7,091,460 claims in situ, dielectric heating using variable radio frequency waves.
In a recent patent publication (U.S. Patent Publication 20070289736/US-A1, filed May 25, 2007), it is disclosed to extract hydrocarbons from a target formation, such as a petroleum reservoir, heavy oil, and tar sands by utilizing microwave energy to fracture the containment rock and for liquification or vitalization of the hydrocarbons.
In another recent patent publication (US Patent Publication 20070131591/US-A1, filed Dec. 14, 2006), it is disclosed that lighter hydrocarbons can be produced from heavier carbon-base materials by subjecting the heavier materials to microwave radiations in the range of about 4 GHz to about 18 GHz. This publication also discloses extracting hydrocarbons from a reservoir where a probe capable of generating microwaves is inserted into the oil wells and the microwaves are used to crack the hydrocarbons with the cracked hydrocarbon thus produced being recovered at the surface.
Despite these disclosures, it is unlikely that direct microwave cracking or heating of hydrocarbons would be practical or efficient. It is known that microwave energy is absorbed by a polar molecule with a dipole moment and bypasses the molecules that lack dipole moment. The absorption of the microwave energy by the polar molecule causes excitation of the polar molecule thereby transforming the microwave energy into heat energy (known as the coupling effect). Accordingly, when a molecule with a dipole moment is exposed to microwave energy it gets selectively heated in the presence of non-polar molecules. Generally, heavy oils comprise non-polar hydrocarbon molecules; accordingly, hydrocarbons would not get excited in the presence of microwaves.
Additionally, while the patent publication above claims to break the hydrocarbon molecules, the energy of microwave photons is very low relative to the energy required to cleave a hydrocarbon molecule. Thus, when hydrocarbons are exposed to microwave energy, it will not affect the structure of a hydrocarbon molecule. (See, for example, “Microwave Synthesis”, CEM Publication, 2002 by Brittany Hayes).
There have been a number of prior proposals set forth for the upgrading of useful fuels from oil shales and tar sands in situ but, for various reasons, none has gained commercial acceptance. One category of such techniques utilizes partial combustion of the hydrocarbonaceous deposits, but these techniques have generally suffered one or more of the following disadvantages: lack of precise control of the combustion, environmental pollution resulting from disposing of combustion products, and general inefficiency resulting from undesired combustion of the resource.
Another category of proposed in situ upgrading techniques would utilize electrical energy for the heating of the formations. For example, in the U.S. Pat. No. 2,634,961 there is described a technique wherein electrical heating elements are imbedded in pipes and the pipes are then inserted in an array of boreholes in oil shale. The pipes are heated to a relatively high temperature and eventually the heat conducts through the oil shale to achieve a pyrolysis thereof. Since oil shale is not a good conductor of heat, this technique is problematic in that the pipes must be heated to a considerably higher temperature than the temperature required for pyrolysis in order to avoid inordinately long processing times. However, overheating of some of the oil shale is inefficient in that it wastes input electrical energy, and may undesirably carbonize organic matter and decompose the rock matrix, thereby limiting the yield. Further electrical in situ techniques have been termed as “ohmic ground heating” or “electrothermic” processes wherein the electric conductivity of the formations is relied upon to carry an electric current as between electrodes placed in separated boreholes. An example of this type of technique, as applied to tar sands, is described in U.S. Pat. No. 3,848,671. A problem with this technique is that the formations under consideration are generally not sufficiently conductive to facilitate the establishment of efficient uniform heating currents. Variations of the electrothermic techniques are known as “electrolinking”, “electrocarbonization”, and “electrogasification” (see, for example, U.S. Pat. No. 2,795,279). In electrolinking or electrocarbonization, electric heating is again achieved via the inherent conductivity of the fuel bed. The electric current is applied such that a thin narrow fracture path is formed between the electrodes. Along this fracture path, pyrolyzed carbon forms a more highly conducting link between the boreholes in which the electrodes are implanted. Current is then passed through this link to cause electrical heating of the surrounding formations. In the electrogasification process, electrical heating through the formations is performed simultaneously with a blast of air or steam. Generally, the just described techniques are limited in that only relatively narrow filament-like heating paths are formed between the electrodes. Since the formations are usually not particularly good conductors of heat, only non-uniform heating is generally achieved. The process tends to be slow and requires temperatures near the heating link which are substantially higher than the desired pyrolyzing temperatures, with the attendant inefficiencies previously described.
Another approach to in situ upgrading has been termed “electrofracturing”. In one variation of this technique, described in U.S. Pat. No. 3,103,975, conduction through electrodes implanted in the formations is again utilized, the heating being intended, for example, to increase the size of fractures in a mineral bed. In another version, disclosed in U.S. Pat. No. 3,696,866, electricity is used to fracture a shale formation and a thin viscous molten fluid core is formed in the fracture. This core is then forced to flow out to the shale by injecting high pressured gas in one of the well bores in which an electrode is implanted, thereby establishing an open retorting channel.
In general, the above described techniques are limited by the relatively low thermal and electrical conductivity of the bulk formations of interest. While individual conductive paths through the formations can be established, heat does not radiate at useful rates from these paths, and efficient heating of the overall bulk is difficult to achieve.